The present invention relates generally to optical modulators and, more particularly, to optical modulators having electrodes proximate waveguides for biasing optical outputs.
Technologies associated with the communication of information have evolved rapidly over the last several decades. Optical information communication technologies have evolved as the technology of choice for backbone information communication systems due to, among other things, their ability to provide large bandwidth, fast transmission speeds and high channel quality. Optical modulators are used to impress data onto optical carriers for transmission through optical fiber.
Compared to their bulk counterparts, guided-wave electro-optic modulators offer a drastic reduction in the required driving power combined with a bandwidth extending well into the 40 GHz band. In an electro-optic modulator, an electric signal is applied to electrodes embedded in a substrate and proximate to the waveguide(s), to induce an electric field which in turn causes, via the electro-optic effect, phase modulation. The latter is typically converted into amplitude modulation by redistributing light among output channels, of which some may correspond to guided modes and others to radiation modes.
One example of a guided-wave, electro-optic modulator is the Mach-Zehnder interferometer (MZI). As shown in FIGS. 1(a) and 1(b), the waveguide 8 of an MZI typically includes an input section 10, a symmetric input Y-junction 12, two interferometer arms 14 and 16, a symmetric output Y-junction 18, and an output section 19. The input Y-junction 12 splits light launched into the input section 10 into two waves propagating through the interferometer arms 14 and 16. The waves are combined and interfere, with a certain phase difference between them, in the output Y-junction 18. The differential phase determines the split of optical power between the fundamental (even, FIG. 1(a)) and first-order (odd, FIG. 1(b)) mode. The output single-mode section 19 supports the fundamental mode but rejects the first-order mode, which is below cut-off and diffracts into the substrate as it propagates towards the output endface of the substrate. If the differential phase equals an even integer of pi, only the fundamental mode is excited in the output Y-junction 18 and propagates, with very little loss, through the output section 19. However, when the differential phase is an odd integer of pi, only the first-order mode is excited in the output Y-junction 18 and subsequently rejected by the output section 19. The differential phase is affected by asymmetries that the interferometer arms 14 and 16 may have, referred to herein as intrinsic bias, as well as by a voltage applied to electrodes (not shown in FIGS. 1(a) and 1(b)) that are disposed proximate the interferometer arms. As discussed in more detail below, with respect to FIG. 2, the voltage applied to these electrodes is used to control the differential phase and achieve amplitude modulation of the optical carrier.
The intrinsic bias of the interferometer is affected by many factors, such as asymmetries (intentional or caused by fabrication tolerances), temperature, surface condition, etc. Of these factors, temperature should be considered in most applications, since typically no temperature stabilization is permitted in commercial, packaged devices. It is known that the temperature dependence of bias is caused by at least two components, one being a surface-state-related bias drift and the other an electrode-introduced stress. The drift component accounts for non-reversible changes in bias that are temperature dependent but do not directly follow changes in temperature. For example, when the modulator goes through the up-ramp of a temperature cycle and subsequently through a symmetric down-ramp, the bias after the cycle does not return to the pre-cycle value and is in fact determined by the whole history of previous temperature changes. On the other hand, the electrode-introduced stress produces, via the elasto-optic effect, a bias component that is uniquely determined by temperature. If, in the above exemplary modulator of FIGS. 1(a) and 1(b), only the stress component of bias would be present, such a modulator could undergo multiple symmetric temperature cycles with its operating point following the same curve during a cycle and returning to the starting point at the end of the cycle.
Accordingly, Applicant would like to provide modulators and methods of making and operating modulators which substantially reduce or eliminate the stress-induced temperature dependent component of bias in such devices.